This invention relates to the inspection of solder joints and more particularly to an apparatus and method for performing automated visual inspection of solder joints on printed circuit boards using surface mount technology (SMT).
Visual inspection of individual solder joints checks for correct mechanical (physical) attributes which are characterized by the topology of the joint and the correct photometric (finish) attributes which are characterized by the reflectivity of the joint. Automated inspection machines in the prior art address either the inspection of photometric attributes or the inspection of mechanical attributes, but not both. Such prior art inspection machines comprises either radiographic methods, X-ray laminographic methods, thermal reflectance methods, visible light spectrum reflectance methods, optical or triangulation methods. However, each of the methods used for inspection have fundamental deficiencies that make them unsuitable for meeting all the inspection requirements in Section 4.4 of Military Standard MIL-STD-2000A, "Standard Requirements for Soldered Electrical and Electronic Assemblies", Department of Defense, USA, which includes not only inspection for defects in the solder joints (mechanical and photometric attributes) but also the condition of the printed circuit board and mounted devices.
Radiographic images represent the x-ray transmittance properties of solder joints. U.S. Pat. No. 4,809,308, issued on Feb. 28, 1989, to John Adams et al. entitled "Method and Apparatus For Performing Automated Circuit Board Solder Quality Inspection" and assigned to IRT Corporation describes a method and apparatus for measuring structural characteristics of a manufactured circuit board containing solder joints by automated real-time digital X-ray radiographic inspection techniques. This technique cannot extract any information of the surface reflectance or topological properties of the joint.
U.S. Pat. No. 4,926,452, issued on May 15, 1990, to Bruce D. Baker et al. entitled "Automated Laminography System for Inspection of Electronics" and assigned to Four PI Systems Corporation, discloses a laminography system for inspecting electrical connections by producing cross-sectional images of the connections which are then analyzed by a computer aided image analysis system. X-ray laminograph images represent the transmittance property of a horizontal cross-section of the solder joint. A multitude of cross-sections would represent the topology properties of the joint. However, the method cannot convey surface reflectance properties of the solder joint.
Thermal reflectance techniques use active methods to detect the cumulative near infrared energy radiated from a solder joint with respect to time. U.S. Pat. No. 4,696,104, issued on Sep. 29, 1987, to Riccardo Vanzetti et al. entitled "Method and Apparatus for Placing and Electrically Connecting Components on a Printed Circuit Board" and assigned to Vanzetti System Inc., describes a system that checks the quality of a solder joint Using an infrared detector to sense the thermal radiation from the heated solder material during cool down which provides a signal for comparison with a standard signal signature. However, no inferences can be made about the precise topology or surface reflectance properties of the joint since an image is not acquired. Also, the technique is unable to identify minor (spatially localized) impurities.
Optical triangulation methods construct a detailed topology of the solder joint. An article entitled "A Three-dimensional Approach to Automatic Solder Inspection" by Sullivan S. Chen, Electronic Manufacturing, November 1988, describes a three-dimensional (3D) vision system manufactured by Robotic Vision Systems Incorporated, using optical triangulation and image processing techniques based on structured light and software having rules based on theoretical and expert knowledge about what makes a joint good or defective. No decision is based on absolute comparisons but on a multitude of measurements and the relationship between them. However, the method is unable to extract reflectance properties of such joints. Also, the extraction of data relies on precise calibration of the printed circuit board plane, without which topology representation would be precarious.
U.S. Pat. No. 4,876,455, issued on Oct. 24, 1989 to Arthur C. Sanderson et al. entitled "Fiber Optic Solder Joint Inspection System" and assigned to Westinghouse Electric Corporation, discloses a method and apparatus for determining the topology of an object having a specular surface using a series of point light sources and associated reflections from the object surface to detect light patterns which are interpreted and used to reconstruct the object surface through curve fitting; a rule-based system through comparison with acceptable solder joint surface features, evaluates and classifies the joint for an acceptable determination. U.S. Pat. No. 4,988,202, issued on Jan. 29, 1991, to Shree K. Nayar et al., entitled "Solder Joint Inspection System and Method" and assigned to Westinghouse Electric Corporation, discloses a similar system as in Sanderson et al., where the surface reflections of the joint are utilized to generate an Extended Gaussian Image representation of the joint which is then evaluated to determine the joint acceptability. However, these techniques are not robust for evaluating photometric attributes because data inference made through curve fitting can overlook small defects.
U.S. Pat. No. 4,688,939, issued on Aug. 25, 1987, to Rajarshi Ray, entitled "Method and Apparatus for Inspecting Articles" and assigned to AT&T Technologies, Incorporated, discloses a method and apparatus for inspecting a chip carrier for the presence of light-reflective solder bumps using a ring lamp, a television camera, and a vision system coupled to the television camera for creating a one-dimensional profile of intensity of the image for each window which is then analyzed to detect for defects by examining the spacing and characteristics of the peaks in such window. However, this system requires a very high resolution camera in order to detect small defects because of its one-dimensional inspection characteristics. It also does not have the capability for printed circuit board inspection, and it only addresses solder bumps, i.e., hemispherical surfaces.
The present invention of an Automated Vision Inspection System is capable of inspecting for all defect categories called for in Section 4.4 of MIL-STD-2000A.